PM says it 'would not undermine or affect in any way' his own marriage.

Malcolm Turnbull has delivered a thinly-veiled smackdown to conservative opponents of marriage equality, asserting that same-sex couples being allowed to marry "would not undermine or affect in any way" his own marriage.

The Prime Minister held court at the National Press Club on Thursday, delivering what might be his final major address before Saturday's election. It was a speech which thickly laid on his stock-standard campaign messages -- jobs, growth, opportunity, stability, "never been a more exciting time to..." -- etc.

It didn't exactly blow the doors off the place. We've heard all this before. But it was the penultimate question from the gathered journalists which saw Turnbull give a pretty excellent answer. Nearing the end of a week where marriage equality and the proposed plebiscite have been front and centre, Sky News host David Speers asked the PM to explain "why you think same-sex marriage should be made legal in Australia".

In response, Turnbull picked apart some of the criticisms of the anti-marriage equality crowd, including the likes of his own party members such as Cory Bernardi and Eric Abetz as well as church groups, who have stated that allowing same-sex couples to get married would undermine or damage the very institution of marriage.

Fairfax Media
Malcolm Turnbull at the National Press Club

"Well, Lucy and I have been married for more than 36 years and we believe that we have no doubt that if same-sex couples were able to describe or formalise their relationship as a marriage, we have no doubt that would not undermine or affect in any way adverse way our relationship, our marriage," Turnbull said.

"The truth of the matter is that the key to marriage is commitment. The threat to marriage is obviously lack of commitment, cruelty, desertion, all of those things."

The PM has long said he supports the plebiscite but would personally vote yes in any such national vote, and backed up that claim in his Press Club address.

"For our part, and I know I can speak on behalf of Lucy and myself here, we will be voting 'yes' in the plebiscite. We completely respect the views of those who will vote 'no', but our view is that we welcome couples making a strong commitment and we are very pleased to support that being described and from a legal point of view, as a marriage," he said.